White Light Emitting Diodes (WLEDs) hold much promise as the number one source of electric light in the future but their acceptance has been plagued by high costs, poor performance and poor reliability. WLED light solutions do exist now but they are priced outside the reach of most households and the product return rate remains stubbornly high.
For low cost applications some designers will try to drive a string of LED lamps directly across the Alternating Current (AC) mains using only a resistor as a ballast. While this strategy is indeed inexpensive it suffers from very low efficiency. The number of WLEDs in the string must be sized small enough so that the sum of all the forward voltage drops is less than the peak AC drive signal, otherwise current will never flow through the diodes and the diodes will never provide any light. If the forward voltage of all the diodes is much less than the peak AC drive voltage then a large amount of power will be dissipated across the ballast resistor and the efficiency of the lamp will be greatly reduced.
If the forward voltage of all the diodes is close to the peak AC voltage then the efficiency will improve but the power factor will degrade. Also, as the AC drive signal changes from high line conditions to low line conditions the amount of current through the diode string changes as will the light output. The current may change enough to put it outside the safe operating range of the diode which will, at the very least, degrade the diode as well as create high temperatures subsequently lowering the life of the WLED string.